The present invention relates to a numerical control (NC) apparatus, and more particularly to an NC method for tapping using a cutting tool or a tapper under synchronized control over a feed shaft and a spindle, and an apparatus therefor.
The tapping process under numerical control is generally conducted by issuing instructions in tapping process cycles in a machining program for a start position, an end position, a tapping feed rate, a tapping spindle rate, etc.
FIG. 1 shows an example of a tapping machining cycle with a tool (tapper) 20, which is generally referred to as a tapping cycle. According to this machining program, a tool or a tapper 20 is moved to a tapping-ready position P.sub.S commanded by the program (action of preparation) and to a tapping start position P.sub.R (the first action). When the action to move the tool 20 to the tapping start position P.sub.R is ended, the feed shaft movement and the spindle rotation are synchronized based on the feed rate and the spindle rate, and the tool 20 is moved to a tapping and position P.sub.e and then returned to the position P.sub.R by reversely rotating the spindle (the third action). When the third action is ended and tool 20 is positioned at the tapping start position P.sub.R, the tool 20 is then moved to a start position P.sub.S for the next machining cycle.
Since the demand has increased for automated assembly processes such as the one for tightening a screw, it is required to set the spindle at a predetermined angle at the tapping start position P.sub.R. In order to meet such a requirement, there has been proposed a tapping cycle which has a second action additionally so that a tapping start angle is command by a machining program, and the spindle angle is set at the command angle at the position P.sub.R.
FIG. 2 is a block diagram to show a prior art embodiment of the NC apparatus which can realize the aforementioned tapping cycle.
A machining program 1 is inputted to an NC apparatus via a tape reader (not shown), and the data of the machining program 1 is read in at a program interpretation section 2 for each block. The program interpretation section 2 analyses the data which is being consequently read in the section of one block, and calculates the first NC command value N.sub.C. The first NC command value N.sub.C comprises generally a G-code, a shaft movement command, a commanded feed rate, a spindle rotation rate, etc. The first NC command value N.sub.C for tapping process comprises a G-code designating a tapping process, a tapping start position P.sub.R, a tapping end position P.sub.e, a tapping start angle .theta..sub.R, a tapping feed rate V.sub.R, a tapping spindle rate W.sub.R, etc. Although the above commands are given in a program in this embodiment, they may be given by a switch such as a panel. The first NC command value N.sub.C which has been calculated by the program interpretation section 2 is sent to a tapping action dividing section 10, and upon receipt of a G-code designating the tapping process, the tapping action dividing section 10 calculates the second NC command value N.sub.C ' which defines the first action for moving the feed rate at the tapping start position P.sub.R, and sends the value to a interpolation section 3. The interpolation section 3 calculates a feed shaft displacement .DELTA.f per unit time and a spindle displacement .DELTA.S per unit time based on the second NC command value N.sub.C, and sends them to a feed shaft controlling section 4 and a spindle controlling section 5, respectively. The controlling section 4 and 5 drive a feed shaft motor 6 and a spindle motor 8 by a feed back control with a feed shaft position detector 7 and a spindle angle detector 9 to execute the first action, respectively.
The tapping action dividing section 10 calculates the second NC command value N.sub.C ' for the second action setting the spindle angle at a tapping start angle .theta..sub.R, and sends the second NC command value N.sub.C ' to the interpolation section 3 when the first action has finished at the interpolation section 3. The actions taken by the interpolation section 3 subsequent thereto are similar to the one mentioned above, and the description will therefore be omitted. The tapping action dividing section 10 calculates a second NC command value N.sub.C ' for the third action for synchronizing the feed shaft with the spindle based on the feed rate V.sub.R and the spindle rate W.sub.R and moving the feed shaft to the tapping end position P.sub.e, and sends the second NC command value N.sub.C ' to the interpolation section 3 when the interpolation section 3 has finished the second action. Subsequent actions of the interpolation section 3 are similar to the first action mentioned above, and the description will be omitted. When the tapping action dividing section 10 receives a G-code other than the G-code designating the tapping process, the tapping action dividing section 10 sends the first NC command value N.sub.C as the second NC command value N.sub.C ' to the interpolation section 3 as is.
FIG. 3 shows an example of the chronological changes in the feed rate V and in the spindle rate W according to the prior art NC apparatus wherein the feed shaft position, the spindle angle, the feed rate and the spindle rate are expressed in the coordinates P(t), .theta.(t), V(t) and W(t) against the time t. The feed rate at the start of the first action is set at V.sub.S and the spindle rate at the start of the second action is set at W.sub.S, but the feed rate at the start of the first action is assumed to be suspended and expressed as V.sub.S =0. At the start of the first action or at t.sub.0, the relationship V(t.sub.0)=V.sub.S =0 holds. At the end of the first action or at t.sub.1, the relationship P(t.sub.1)=P.sub.R, V(t.sub.1)=0 holds. At the end time t.sub.1 of the first action, the second action starts and the relationship W(t.sub.1)=W.sub.S holds. At the end time point t.sub.2 of the second action, the relationship as .theta.(t.sub.2)=.theta..sub.R, W(t.sub.2)=0 holds. The third action starts at the end point t.sub.2 of the second action. The maximum feed rate V.sub.A of the first action may be commanded as the tolerable maximum feed rate in the machining program 1 or may be incorporated as an eigenvalue to the machine within the NC apparatus. It becomes possible to set the spindle angle at the tapping start position P.sub.R constantly at a commanded value in a prior art NC apparatus such as that described above.
However, for each of a tapping process, it is necessary in the above prior art device to conduct the second action after having the first action end or more particularly after having the feed shaft movement suspended. As shown in FIG. 3, every time a tapping process is conducted, an action comprising the steps of (a) portion reducing the feed rate, (b) portion suspending the feed shaft for the time period calculated by (t.sub.2 -t.sub.1), and (c) portion accelerating the feed shaft rate is required.
In tapping processes, a tool is generally used to tap threads in plural screws continuously in order to prevent time loses which would otherwise be caused by replacing tools. The necessity to conduct the second action of reduction, suspension and acceleration of the speed of the feed shaft arises every time a tapping process ends to thereby inconveniently increase the required machining time. More particularly, even if the tapping machining process is added to a function to set the spindle angle at the start point to be a predetermined angle by command, the apparatus needs to follow the processes comprising the first action to move the feed shaft at a tapping start position by a command in advance, the second action to set the spindle angle at the predetermined tapping start angle, and the third action to synchronize the feed shaft and the spindle based on the preinstructed feed rate and the spindle rate; but each of the actions had to wait for the preceding action to have finished before starting to thereby increase the processing time inconveniently.